1. Field of the Invention
The present invention relates to a system and method for assessing capillary vitality. In particular, the present invention relates to an instrument for the automated measurements of skin perfusion pressure and tissue gases in a local or regional body site to allow a clinician to predict the ability of a wound to heal. The tissue gases principally include CO2, but numerous other gases may also be measured.
2. Description of the Related Art
Transcutaneous oximetery (TCOM) is a commonly used tool for vascular assessment. TCOM is uniformly used to assess the severity of arterial blockage; assess the potential for healing of wounds, ulcers and amputations; evaluate the outcome of a revascularization procedure; and assess the severity and progression of peripheral vascular disease. TCOM measurements are conducted in a variety of ways. One measurement device uses a polarographic sensor, a silver anode, an electrolyte, an oxygen permeable membrane and a heating section to control temperature. When the sensor, placed near the measurement site, detects oxygen, it causes an electrochemical reaction that results in an electrical current to flow through the cathode. An amplifier connected to the cathode measures the amount of current and correlates it to the amount of oxygen reaching the skin. This information is displays as TcPO2. The clinician uses the TcPO2 value to assess the ability of the wound, ulcer, or amputation to heal, the success of revascularization, and otherwise predict the behavior of healing. However, problems abound using this conventional device as the standard predictor for wound healing. This is because while oxygen can be present at the measurement site, the presence of oxygen is not necessarily a predictor of a tissue's ability to heal. Thus a very low TcPO2 level is thought to be predictive of a wound not having the ability to heal. However, a clinician evaluating a high or normal TcPO2 would conclude that the wound does have the ability to heal when that is not necessarily the case. This is especially true with respect to lower extremity ulcers. (See Faglia et al Angiographic Evaluation of PAOD and its Role as a Prognostic Determinant for Major Amputation in Diabetic Subjects with Foot Ulcers. Diabetic Care 1998; 21:4; 625.)
Pulse oximeters are another device that monitor the oxygenation in a patient's arterial blood. Conventional pulse oximeters measure the ratio of changing absorbance of red and infrared light caused by the difference in color between oxygen-bound (bright red) and oxygen unbound (dark red or blue, in severe cases) blood hemoglobin at the measuring site. However, oxygen may reach the measurement site either as it is presently being bound to hemoglobin or unbound from hemoglobin thus skewing the predictive value. Falsely high or low readings will occur when hemoglobin is bound to something other than oxygen such as carbon monoxide. Also, the conventional pulse oximeter is dependant on pulsatile flow so where flow is sluggish, as is frequently the case with wounds and ulcers, the pulse oximeter is less able to provide accurate readings. Pulse oximeters measure only oxygenation; they do not measure carbon dioxide levels, blood pH or sodium bicarbonate levels. Further, although pulse oximetry is used to monitor oxygenation, it cannot determine the metabolism of oxygen and other gases being used by the patient. For this purpose, tissue carbon dioxide levels must be measured. It is also known that oximetry is not a complete measure of circulatory sufficiency. As pointed out above, if there is insufficient blood flow or insufficient hemoglobin in the blood, such as patients with anemia, tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive to the wound site.
Other measurement devices such as hyperspectral imaging which tracks chromophores and thermal imaging which detects heat secondary to blood flow, do not assess the ability of a capillary bed to overcome resistance, a mechanical factor. Hyperspectral imaging is built on the concept that tissues have optical signatures or chromophores that reflect their chemical characteristics. The two major chromophores of physiological relevance in hyperspectral imaging are oxyhemoglobin (OxyHb) and deoxyhemoglobin (DeoxyHb). When measured by hyperspectral imaging, these chromophores delineate local oxygen delivery and extraction within the tissue microvasculature. However, detecting the presence or absence of oxygen does not take into account the functional assessment of the capillary bed, which can only be done by creating a challenge environment in which capillary flow is monitored during the application of occlusive pressure and controlled release of that pressure. This type of measurement is termed “Skin Perfusion Pressure” (SPP). These measurements are informative to the physician because fluid movement in the capillaries is possible in low flow states but the opening pressure required by the capillaries to overcome occlusion may not be sufficiently adequate to maintain tissue health.
In damaged tissues there is a marked difference in capillary performance. For example, the capillary density of the skin of the foot is significantly reduced in patients with arterial ulceration compared with that in patients with claudication and healthy subjects. (See e.g. Lamah, Mortimer, Dormandy. Qualitative study of capillary density in the skin of the foot in peripheral vascular disease. British Journal of Surgery 86(3): 342-348, 1999.) How this process evolves is likely more a function of damage that is manifest earlier in the release of downstream mediators and mechanisms that result in irreversible tissue injury.
The assessment of tissue carbon dioxide concentration has also been useful in perfusion assessment. See e.g. Fries et al. “Increases in tissue PCO2 during circulatory shock reflect selective decreases in capillary blood flow,” Crit. Care Med. 2006; 34:446-452. At predictable tissue CO2 levels and durations, tissue damage is compromised. Carbon dioxide production, which is associated with metabolism, continues in tissues even during conditions of low blood flow. The concentration of carbon dioxide increases in tissues experiencing low blood flow because the carbon dioxide is not rapidly carried away. This carbon dioxide increase (measured as an increase in partial pressure of CO2 (PCO2) in turn results in a decrease in pH in nearby tissue. Therefore, the ability of a tissue to heal may be assessed by measuring pH or PCO2 at these sites.
Significantly, one may have blood flow at a site (and conventional methods will detect a near normal to normal reading of O2) but accumulated CO2 is not cleared secondary to ongoing production of CO2 such as that attributed to necrotic tissues. Therefore, a physician assessing tissue viability may draw an erroneous conclusion thereby compromising patient health. Notably, when limb hypoxia is due to ischemia (low blood flow) PCO2 levels significantly increase. When limb hypoxia is due to hypoxemia (maintained blood flow) PCO2 levels remain constant. However, when blood flow is maintained and a normal oxygen supply is available but PCO2 levels remain constant, then other sources of carbon dioxide production are suspected. Viewed another way, increased tissue carbon dioxide is always associated with compromised tissue health. This is very well illustrated by the significant increases in PCO2 levels accompanied by decreased tissue blood flow seen in septic states. These considerations argue for assessing both mechanical and metabolic factors to accurately assess capillary vitality in low and high blood flow situations.
Thus, a system is needed that overcomes the problems associated with using conventional measurements such as TCOM, the measurement of bound or unbound hemoglobin, thermal imaging and/or hyperspectral imaging to assess wound healing potential. A system is needed that can accurately assess wound healing potential in cases where blood flow is sluggish and in cases where blood flow is high; and one that does not rely solely on oxygen measurements as the predictor of a tissue's ability to heal. Such a system would be a marked improvement over conventional systems and would allow physicians to assess wound healing potential, monitor pre- and post-surgical therapy, assist in arterial reconstruction planning and post-procedure monitoring, amputation planning and post-procedure monitoring; and assist in the diagnosis of peripheral arterial disease, critical limb ischemia and other microcirculatory derangements.